Nd-Doped Fiber Laser Generates 360-fs Pulses at 900 nm

Breck Hitz

Femtosecond lasers are useful in machining applications as well as in medicine, precise rangefinding and fluorescence studies. Fiber lasers offer an attractive alternative to conventional femtosecond sources because of their high efficiency and their potential to generate very high output powers.

A research group at Tampere University of Technology in Finland has focused considerable effort recently on developing ultrafast fiber lasers. In its collaborations with Fianium-NewOptics Ltd. of Southampton, UK, the group has developed what it believes to be the shortest-wavelength mode-locked fiber laser yet reported. It's a passively mode-locked neodymium-doped fiber laser, oscillating on the ~900-nm ⁴F_3/2 – ⁴I_9/2 transition and producing 360-fs pulses.

The fiber laser resonator comprises a semiconductor saturable absorber mirror and a fiber-loop mirror that is 92 percent reflective (Figure 1). A pair of gratings provide anomalous dispersion to compensate for the dispersion in the fiber so that the total intracavity dispersion is anomalous, and the mode-locking, self-starting. The gratings' reflectivity is polarization-dependent, so the researchers use an intracavity controller to adjust the polarization for minimum loss. The gain medium is a meter-long Nd-doped single-mode fiber whose ends are angle-cleaved to avoid intracavity etalons. A wavelength division multiplexer couples up to 100 mW of 809-nm pump power from a diode laser into the fiber.

The output pulses have an average power of 1 mW, and an autocorrelation measurement shows their duration to be 620 fs. However, dispersion in the 18-cm output fiber pigtail accounts for a significant portion of this duration. Longer pigtails result in longer pulses, and by extrapolating backward to zero pigtail length, the researchers calculate that the duration of pulses emerging from the laser is 360 fs. Because the measured bandwidth (FWHM) of the pulses is 2.4 nm, the time-bandwidth product is 0.32, indicating that the output pulses are transform-limited.

The output wavelength of the laser is tuned by tilting the diffraction grating or by adjusting the lens between the grating and the semiconductor saturable absorber mirror. The tuning range is 894 to 909 nm.

Subsequent to the publication of the work described above, the researchers have improved the semiconductor saturable absorber mirror to the point that the bulky gratings can be removed from the resonator, and stable, self-starting mode-locking is achieved with the mirror alone. They have demonstrated that the use of these devices with high modulation depth (>10%) is particularly suitable for fiber lasers and allows for compact mode-locked lasers with a fiber cavity free from dispersion compensators. This approach results in a compact and highly practical picosecond fiber source.

An appropriate dispersion delay line at the output of the master source can be used for de-chirping and producing high-quality pulses. Pulse width can be further reduced using amplification, nonlinear spectrum broadening and pulse compression performed externally to the master oscillator, leading to a cost-effective femtosecond system.